JPH11221886A - Film for stretch packaging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a film for stretch packaging excellent in transparency, flexibility and stretchability and good in self-adhesiveness and recovery properties. SOLUTION: Outer layers comprising a crystalline ethylenic copolymer resin are laminated on both surfaces of an intermediate layer comprising a polypropylene resin. The polypropylene resin used in the intermediate layer has a wt. average mol.wt. of 150,000-700,000 and the elution amt. thereof with respect to the whole of the resin measured by a cross fractionation method is 30-70 wt.% at 0 deg.C or lower, 0-30 wt.% at temp. of 0-90 deg.C and 15 wt.% or less at an arbitrary temp. width of 5 deg.C within this temp. region and 5-40 wt.% at a temp. of 90-130 deg.C. Further, the m.p. difference between the polypropylene resin and the crystalline ethylenic copolymer resin is 30 deg.C or higher.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stretch wrapping film which has suitability for stretch wrapping, has excellent transparency, and can be suitably used for tray wrapping.

[0002]

2. Description of the Related Art Stretch wrapping films are used for wrapping various products, and are particularly suitably used for tray wrapping of foods such as fruits and vegetables and fresh foods (fish, meat, prepared foods, etc.). To tray-pack foods or the like using this stretch-wrapping film, the food to be packaged is usually placed in a plastic container made of styrofoam or the like, the stretch-wrapping film is put on the container, and the film end is pulled. Wrap the container while stretching. In order to enhance the commercial value of the packaged food, a tight and beautiful package appearance is required, and the stretch packaging film must have excellent self-adhesiveness and resilience in addition to transparency, flexibility and extensibility. Required. Conventionally, as a film for stretch packaging used for packaging and the like, a film mainly composed of plasticized polyvinyl chloride (hereinafter, referred to as plasticized PVC) has been widely used in terms of cost and ease of use. In recent years, environmental pollution problems have been emphasized, and the use of plasticized PVC containing chlorine has been regarded as a problem in all fields.

[0003] For this reason, recently, as an alternative to plasticized PVC, the development of stretch packaging films using polyolefin resins has been actively promoted. Examples of such a film for stretch packaging include films made of a linear low-density polyethylene resin, an ethylene-vinyl acetate copolymer (hereinafter, referred to as EVA), and polybutadiene. However, stretch wrapping films using polyolefin-based resins have the problem that the properties required for stretch wrapping films, such as self-adhesiveness and deformation recovery during packaging, cannot be obtained. There was a problem that the film was immediately detached from the container or the film was wrinkled, which was not suitable for actual use. In order to solve such a problem, for example, as described in JP-A-61-44635, EVA is coated on both sides of a layer mainly composed of a random copolymer of propylene and various α-olefins. A film for stretch packaging in which a layer containing a main component is laminated has been proposed, or as described in JP-A-62-51440, a polymer of an aromatic hydrocarbon such as styrene, and a conjugated diene elastomer. There has been proposed a stretch packaging film in which a layer mainly containing EVA is laminated on both sides of a layer mainly containing a block copolymer.

[0004] However, in stretch packaging films made of these copolymers, those having a low recovery rate after stretch packaging cause wrinkling after packaging, and films having a high recovery rate cause rapid recovery. Therefore, there is a problem that the container is deformed, and in any case, there is a problem that it cannot be suitably used as a film for stretch packaging. Also, when using other special elastomers, even if the performance is improved, the cost is high,
It may not be practical. On the other hand, JP-A-6-2565
No. 39, weight average molecular weight is 80,000 or more and 50
The resin elution amount measured by the cross fractionation method is 20% by weight or more and 80% by weight or less at 10 ° C;
5% by weight or more and 45% by weight or less at 10 ° C to 65 ° C;
From 1% by weight to 18% by weight at 65 ° C to 90 ° C;
A stretch packaging film composed of a polypropylene resin having a weight percentage of 10% to 35% by weight at 0 ° C. to 125 ° C. is disclosed. Japanese Patent Application Laid-Open No. 7-40519 discloses a film having a weight average molecular weight of 15 to 50. Ten thousand
The elution amount with respect to the total amount of the polypropylene resin measured by the cross fractionation method is 30 to 60% by weight at 0 ° C or lower, 30 to 55% by weight at 0 ° C or higher and 90 ° C or lower, and any 5% in this temperature range. The density is 0.92% on both surfaces of a center layer made of a polypropylene resin which is 15% by weight or less in a temperature range of 90 ° C and 5% to 20% by weight at 130 ° C or more and 90 ° C or less.
A stretch packaging film comprising a multilayer film in which an outer layer made of a linear low-density polyethylene of 0 g / cm ³ or less is laminated is disclosed.

However, at present, a stretch packaging film that sufficiently satisfies the demands of the market as a substitute for a stretch packaging film containing plasticized PVC as a main component has not yet been commercialized.

[0006]

SUMMARY OF THE INVENTION Accordingly, the present invention solves the problems of the prior art as described above, and is excellent in transparency, flexibility and extensibility, and has good self-adhesiveness and resilience. It is intended to provide a packaging film.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies to achieve the above object, and as a result, an outer layer made of a crystalline ethylene-based copolymer resin was laminated on both surfaces of an intermediate layer made of a polypropylene-based resin. It has been found that the above-mentioned object can be achieved by a stretch-wrapping film having the following characteristics, and the polypropylene-based resin has reached the present invention. (1) The weight average molecular weight is 150,000 to 700,000. (2) The elution amount measured by the cross fractionation method is 30% by weight or more and less than 70% by weight at 0 ° C or less and 0% by weight or more and 30 ° C or less and 0% by weight or more with respect to the whole resin. % And 15% by weight or less at an arbitrary temperature range of 5 ° C. in this temperature range, and more than 90 ° C. and 130 ° C. or less,
5% by weight or more and less than 40% by weight.

(3) The difference in melting point between the polypropylene resin and the crystalline ethylene copolymer resin is 30 ° C. or more.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The film for stretch packaging of the present invention is provided on both surfaces of an intermediate layer made of a polypropylene resin.
An outer layer made of a crystalline ethylene-based copolymer resin is laminated. To construct this sandwich laminate,
For example, a method is generally used in which the raw material resin of each layer is formed into a film by a coextrusion inflation method, and an outer layer made of a crystalline ethylene-based copolymer resin is laminated on both sides of an intermediate layer made of a polypropylene-based resin. The film formation by the coextrusion inflation method is usually carried out by gradually cooling the obtained three-layered film while maintaining the film thickness ratio, for example, 1/1/1 to 1/8/1 and the blow-up ratio. ,
For example, it is appropriate to carry out at 2 to 10, preferably 2.5 to 5. The thickness of the obtained stretch packaging film is, for example, 6 to 30 μm, preferably 10 to 20 μm.
Is appropriate.

In the film for stretch packaging, transparency is the most important quality item, and the haze value is required to be 2% or less, preferably 1% or less. However, unlike the T-die method in which the film can be rapidly cooled, the film is gradually cooled in the inflation method, so that it is extremely difficult to reduce the transparency to a haze value of 1% or less. The present inventors have studied various methods for improving transparency irrespective of the production method, and have completed the present invention. As the polypropylene-based resin used in the present invention, various polypropylene-based resins can be used as long as they have the above-mentioned specific weight-average molecular weight and the amount eluted by the cross fractionation method. As such a polypropylene resin, for example, a resin produced by the following multistage polymerization can be used.

In such a multi-stage polymerization, first, in the first-stage polymerization, a titanium-containing polyolefin is formed in the presence of a titanium compound and an aluminum compound. The titanium-containing polyolefin obtained here is a propylene homopolymer, a propylene-ethylene copolymer or a propylene-α-olefin copolymer. The above α-
Preferred examples of the olefin include 1-butene. Next, in the polymerization after the second step, in the presence of the titanium compound and the aluminum compound, propylene and ethylene are copolymerized with the titanium-containing polyolefin, or propylene and α-olefin are copolymerized with the titanium-containing polyolefin. Let it. by this,
A propylene-ethylene copolymer or a propylene-α-olefin copolymer is obtained. In the obtained polypropylene-based resin, ethylene-propylene copolymer,
In a state where a rubber component such as an ethylene-α-olefin copolymer is uniformly and finely dispersed in a crystalline propylene (co) polymer in an amount of, for example, 40 to 80% by weight, or preferably 50 to 70% by weight. Existing.

The polypropylene resin used for forming the intermediate layer used in the present invention has a weight average molecular weight of 150,000 to 700,000, and the elution amount measured by the cross fractionation method at 0 ° C. or lower is 30% or less based on the total amount of the resin. % By weight and less than 70% by weight, more than 0 and less than 90 ° C, less than 0% by weight and less than 30% by weight, and 1 at an arbitrary temperature range of 5 ° C in this temperature range.
5% by weight or less and less than 40% by weight at 90 ° C. or more and 130 ° C. or less, and the melting point difference between the polypropylene-based resin and the crystalline ethylene-based copolymer resin is 30% by weight or less. C. or higher. If the weight average molecular weight of the polypropylene resin in the intermediate layer is less than 150,000, the stretchability of the obtained stretch packaging film is not sufficient, and if it exceeds 700,000, the flexibility of the obtained stretch packaging film is insufficient. Is not enough. The preferred weight average molecular weight is 300,000 to 600,000, particularly preferably 350,000 to 550,000.

In the present invention, the weight average molecular weight represents a value converted to a propylene homopolymer having an isotactic index of 98.0% or more. The cross fractionation method is known, and the measurement conditions employed in the cross fractionation method in the present invention are as follows. Using a cross separation device (CFC-T150A type) manufactured by Mitsubishi Yuka Corporation, 0.5 ml of a polypropylene resin is injected at a concentration of 0.3 w / v% using ortho-dichlorobenzene as a solvent. The column volume of the TREF (temperature rising fractionation) part is 0.83 ml. The solution is cooled at a constant rate to form a thin polymer layer on the surface of the previously prepared inert element. Under the conditions for forming the polymer layer, the temperature was lowered for 135.0 minutes (from 135 ° C. to 0 ° C.), and then maintained for 30.0 minutes. The flow rate of the solvent in the feed system is 1.0 ml /
The elution temperature was 0 ° C, 20 ° C, and exceeded 20 ° C for 9 minutes.
At 5 ° C or less, intervals of 5 ° C, 98 ° C, over 98 ° C and 135 ° C
Below ℃, it was measured at 3 ℃ intervals (27 categories). The elution volume at this time was 2.0 ml. In the SEC section, the measurement temperature was 135 ° C, two Shodex columns UT806M, UT80
7 and one precolumn UT-G, and the IR detector was measured using MIRAN-1A, 3.42 μm, manufactured by FOXBORO.

In the present invention, the elution amount measured by the cross fractionation method is 30% by weight or more and less than 70% by weight, preferably 35% by weight or more and 60% by weight or less, based on the total amount of the resin.
% By weight. If the elution amount is less than 30% by weight, the obtained stretch packaging film becomes less flexible, and if it exceeds 70% by weight, the elasticity of the obtained stretch packaging film becomes insufficient. When the temperature exceeds 0 ° C. and 90 ° C. or less, the elution amount measured by the cross fractionation method is 0% by weight or more and less than 30% by weight, preferably 10% by weight or more and less than 30% by weight based on the total amount of the resin, In addition, at any 5 ° C. temperature range in this temperature range, 1
It is at most 5% by weight, preferably at most 10% by weight. When the elution amount is more than 0 ° C. and 90 ° C. or less and 30% by weight or more, or if it exceeds 15% by weight at an arbitrary temperature range of 5 ° C., the transparency and deformation recovery property of the obtained stretch packaging film. Is inferior.

Further, when the temperature exceeds 90 ° C. and 130 ° C. or less, the elution amount measured by the cross fractionation method is based on the total amount of the resin.
The content is 5% by weight or more and less than 40% by weight, preferably 7% by weight or more and less than 35% by weight. If the elution amount is less than 5% by weight, the strength and elasticity of the obtained stretch packaging film is not sufficient, and if it is more than 40% by weight, the deformation recovery property of the obtained stretch packaging film is inferior. Differential scanning calorimeter (DSC) for the polypropylene resin in the middle layer
Is usually 130 to 165 ° C., preferably
Suitably it is between 135 and 155 ° C. As the crystalline ethylene copolymer resin of the outer layer, for example, a copolymer of ethylene and an α-olefin having 3 to 12 carbon atoms, or a copolymer of ethylene and vinyl acetate is preferably exemplified. Can be

In the case of a copolymer of ethylene and an α-olefin having 3 to 12 carbon atoms, the density is 0.880 to 0.920 g / c.
m is ^3, preferably, suitably to be 0.890 ~0.910g / cm ^3. As a copolymer of ethylene and an α-olefin having 3 to 12 carbon atoms, a copolymer obtained by polymerization with a metallocene catalyst is preferable. The density of this copolymer is 0.920 g / cm ³
Exceeding the range is not preferred because the resulting stretch packaging film tends to have reduced transparency, flexibility and extensibility. On the other hand, in the case of a copolymer of ethylene and vinyl acetate,
The vinyl acetate content is 3 to 30% by weight, preferably 5 to 2%.
Suitably, it is 5% by weight. The melting point of the crystalline ethylene copolymer resin determined by differential scanning calorimetry (DSC) is
For example, 75 to 120 ° C, preferably 80 to 110 ° C
It is appropriate that

The difference between the melting points of the crystalline ethylene copolymer resin and the polypropylene resin is at least 30 ° C.
Preferably, it is 40 ° C., particularly preferably 50 ° C. Note that the maximum of the melting point difference will usually be 90 ° C.
The larger the melting point difference, the more excellent the heat sealing property is provided, and the better the heat sealing workability. The stretch packaging film of the present invention uses a specific polypropylene-based resin as an intermediate layer and a specific crystalline ethylene-based copolymer resin as an outer layer, and thus has excellent transparency, flexibility, and extensibility. It has good self-adhesiveness and resilience.

[0018]

The present invention will be described below with reference to examples. Example 1 A polypropylene resin having the following characteristics was used as an intermediate layer. (1) The weight average molecular weight is 451,000. (2) The elution amount based on the total amount of the polypropylene resin measured by the cross fractionation method is 56.3% by weight at 0 ° C., 22.5% by weight at more than 0 ° C. and 90 ° C. or less, and an arbitrary temperature range of 5 ° C. in this temperature range. 15% by weight or less, and
It is 21.2% by weight above 90 ° C and below 130 ° C. (3) Density 0.880 g / cm ³ .

(4) MFR 0.5. (5) melting point 140 ° C. On the other hand, as the resin for the outer layer, a linear low-density polyethylene resin (ethylene-octene copolymer polymerized by a metallocene catalyst manufactured by Dow Chemical Company, Affinity FW 1650, density
Using 0.902 g / cm ³ , MI 3.0 (190 ° C., melting point 100 ° C.), 100 parts by weight of this resin was added to an antifogging agent (manufactured by RIKEN VITAMIN CO., LTD.
EAR-2) 25 parts by weight were added. The above-mentioned polypropylene resin and linear low-density polyethylene resin are gradually cooled by a co-extrusion inflation method, and while being gradually cooled, a film for stretch packaging (blowing) having a sandwich-like laminated structure in which outer layers are provided on both sides of a single intermediate layer. Up ratio 3.5,
A total thickness of 15 μm and a thickness ratio of 1/6/1) were prepared. Example 2 Instead of the resin for the outer layer in Example 1, the vinyl acetate content was 18% by weight, the melt flow rate was 2.5 g / 10 minutes,
Example 1 was repeated except that an ethylene-vinyl acetate copolymer resin having a melting point of 82 ° C was used. <Measurement Method> - breaking strength: at a pressure 2.0 kg / cm ^2, for 1 second: JIS Z1702, elongation at break: JIS Z1702 - Haze: JIS K1705-Gloss: JIS K1705-Clarity: JIS K1705, the heat sealing temperature The temperature at which the seal strength starts to take a nearly constant value. -Suitability for stretch packaging: ISHIDA Wmi manufactured by Automatic Packaging Machine (Ishida Hoki Co., Ltd.) using a stretch film with a width of 350 mm
n Expanded polystyrene tray (200 mm long, MK-II)
130 mm wide and 30 mm high) were packaged and evaluated.

The results are shown in Table 1 below. As is clear from Table 1, the stretch packaging film of the example is
Good results were shown for all items.

[0021]

Table 1 Example 1 Example 1 Example 2 Breaking strength (kg / cm ² ) Vertical 350 320 Horizontal 180 170 Elongation at break (%) Vertical 520 470 Horizontal 670 700 Haze (%) 0.9 0.9 Gloss (%) 132 131 Clarity (%) Vertical 86 84 Horizontal 81 81 Heat sealing temperature (℃) 105 87 Suitability for stretch packaging Suitable If the difference in melting point between the intermediate layer and the outer layer is less than 30 ° C, the seal is clean during heat sealing. Because it is difficult to come out
It became a film for stretch wrapping with poor practicality.

[0022]

According to the present invention, it is possible to provide a stretch packaging film which is excellent in transparency, flexibility and extensibility, and has good self-adhesiveness and resilience.

Claims (1)

[Claims] 1. A stretch packaging film in which an outer layer made of a crystalline ethylene-based copolymer resin is laminated on both surfaces of an intermediate layer made of a polypropylene-based resin, wherein the polypropylene-based resin has the following characteristics: A film for stretch packaging characterized by having: (1) The weight average molecular weight is 150,000 to 700,000. (2) The elution amount measured by the cross fractionation method is 0% or less and 30% by weight or more and less than 70% by weight, and 0 ° C or more and 90 ° C or less and 0% by weight or more with respect to the whole resin. Less than 15% by weight at an arbitrary temperature range of 5 ° C. in this temperature range, and 5% by weight or more and 40% by weight at 90 ° C. or more and 130 ° C. or less.
Is less than. (3) The difference in melting point between the polypropylene resin and the crystalline ethylene copolymer resin is 30 ° C. or more.